1. Field of the Invention
The present application discloses a microporous membrane and a method for making the same.
2. Description of the Prior Art
A microporous membrane comprising a very high molecular weight polyolefin and an inert filler material was taught by Larsen, U.S. Pat. No. 3,351,495. The general principles and procedures of U.S. Pat. No. 3,351,495 are incorporated herein by reference.
Kono et al., U.S. Pat. No. 4,600,633 teaches a polyethylene superthin film and a process for the production of the same. In this process an ultra high molecular weight polyethylene (herein after UHMWPE) is dissolved in a solvent then extruded to form a gel sheet. The gel sheet then undergoes a first extraction step to remove the solvent. After the first extraction, the sheet is heated and stretched. The stretched sheet then undergoes a second extraction step to remove solvent. The resulting product then undergoes a compression treatment at a temperature of 80° to 140° centigrade. This reference does not use a filler in its UHMWPE. The gel sheet is not calendered prior to solvent extraction. The resulting product is a thin film with a tensile modulus of at least 2000 kg/cm2 a breaking strength of at least 500 kg/cm2 and which, is substantially free from pores.
Schwarz et al., U.S. Pat. No. 4,833,172 teaches a stretched microporous material. In this process an UHMWPE and a siliceous filler are dissolved in a plasticizer then extruded to form a gel sheet. In this process the gel sheet may optionally be calendered prior to a solvent extraction. The gel sheet then undergoes a solvent extraction to remove the plasticizer. After the extraction, the sheet is then stretched.